The long-term goal is to understand the genetic control of development in a simple animal. The small, free-living soil nematode, Caenorhabditis elegans is a superior organism for a genetic study of development. The structure of the animal is precisely specified by its genes. Its small size and simplicity make possible the electron microscopic study of development to a completeness impractical with most other organisms. The ability to select and characterize mutants affected in specific developmental pathways can provide the basis for unique contributions to the understanding of developmental control mechanisms common to all animals. The experimental approach employs genetic, microscopic and biochemical methods to a detailed analysis of a particular "developmental switch" in the life cycle. The dauer larva is a non-growing, non-feeding stage which is formed under conditions of starvation, possesses a specialized resistant cuticle, and differs from all other larval stages in behavior and morphology. Dauer larvae can survive harsh conditions for months, but when favorable conditions are encountered, they recover to resume development and produce offspring. This developmental pathway offers a number of advantages for genetic study. The specific goal is to determine the complete genetic program for dauer larva formation and recovery, and the molecular mechanisms for the implementation of that program.